add Docker deployment with Alpine image, release config and health check

- Alpine multi-stage Dockerfile (131 MB image)
- Release overlays (bin/server, bin/migrate), env.sh, Release module
- Health check endpoint at GET /health
- Fly.io config with SQLite volume mount
- Fix hardcoded paths in optimizer.ex and variant_cache.ex to use
  Application.app_dir/2 (breaks in releases where Plug.Static serves
  from a different directory than CWD)
- strip_beams: true in release config
- Optimised .dockerignore and .gitignore for mockup variants

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/simpleshop_theme/images/optimizer.ex

@@ -13,11 +13,10 @@ defmodule SimpleshopTheme.Images.Optimizer do
   # Only affects <5% of users (legacy browsers without AVIF/WebP support)
   @pregenerated_formats [:avif, :webp]
   @thumb_size 200
-  @cache_dir "priv/static/image_cache"
   @max_stored_width 2000
   @storage_quality 90
 
-  def cache_dir, do: @cache_dir
+  def cache_dir, do: Application.app_dir(:simpleshop_theme, "priv/static/image_cache")
   def all_widths, do: @all_widths
 
   @doc """
@@ -72,7 +71,7 @@ defmodule SimpleshopTheme.Images.Optimizer do
         {:ok, :svg_skipped}
 
       %{data: data, source_width: width} = image ->
-        File.mkdir_p!(@cache_dir)
+        File.mkdir_p!(cache_dir())
 
         with {:ok, vips_image} <- Image.from_binary(data) do
           widths = applicable_widths(width)
@@ -93,7 +92,7 @@ defmodule SimpleshopTheme.Images.Optimizer do
   end
 
   defp generate_thumbnail(image, id) do
-    path = Path.join(@cache_dir, "#{id}-thumb.jpg")
+    path = Path.join(cache_dir(), "#{id}-thumb.jpg")
 
     return_if_exists(path, fn ->
       with {:ok, thumb} <- Image.thumbnail(image, @thumb_size),
@@ -104,7 +103,7 @@ defmodule SimpleshopTheme.Images.Optimizer do
   end
 
   defp generate_variant(image, id, width, format) do
-    path = Path.join(@cache_dir, "#{id}-#{width}.#{format_ext(format)}")
+    path = Path.join(cache_dir(), "#{id}-#{width}.#{format_ext(format)}")
 
     return_if_exists(path, fn ->
       with {:ok, resized} <- Image.thumbnail(image, width),
@@ -140,12 +139,12 @@ defmodule SimpleshopTheme.Images.Optimizer do
   """
   def disk_variants_exist?(image_id, source_width) do
     widths = applicable_widths(source_width)
-    thumb = File.exists?(Path.join(@cache_dir, "#{image_id}-thumb.jpg"))
+    thumb = File.exists?(Path.join(cache_dir(), "#{image_id}-thumb.jpg"))
 
     variants =
       Enum.all?(widths, fn w ->
         Enum.all?(@pregenerated_formats, fn fmt ->
-          File.exists?(Path.join(@cache_dir, "#{image_id}-#{w}.#{format_ext(fmt)}"))
+          File.exists?(Path.join(cache_dir(), "#{image_id}-#{w}.#{format_ext(fmt)}"))
         end)
       end)
 
@@ -159,12 +158,12 @@ defmodule SimpleshopTheme.Images.Optimizer do
   """
   def generate_variant_on_demand(image_data, image_id, width, format)
       when is_binary(image_data) and format in [:avif, :webp, :jpg] do
-    path = Path.join(@cache_dir, "#{image_id}-#{width}.#{format_ext(format)}")
+    path = Path.join(cache_dir(), "#{image_id}-#{width}.#{format_ext(format)}")
 
     if File.exists?(path) do
       {:ok, path}
     else
-      File.mkdir_p!(@cache_dir)
+      File.mkdir_p!(cache_dir())
 
       with {:ok, vips_image} <- Image.from_binary(image_data),
            {:ok, resized} <- Image.thumbnail(vips_image, width),

lib/simpleshop_theme/images/variant_cache.ex

@@ -19,7 +19,7 @@ defmodule SimpleshopTheme.Images.VariantCache do
   alias SimpleshopTheme.Sync.ImageDownloadWorker
   import Ecto.Query
 
-  @mockup_dir "priv/static/mockups"
+  defp mockup_dir, do: Application.app_dir(:simpleshop_theme, "priv/static/mockups")
 
   def start_link(opts) do
     GenServer.start_link(__MODULE__, opts, name: __MODULE__)
@@ -87,9 +87,9 @@ defmodule SimpleshopTheme.Images.VariantCache do
   end
 
   defp ensure_mockup_variants do
-    if File.dir?(@mockup_dir) do
+    if File.dir?(mockup_dir()) do
       sources =
-        Path.wildcard(Path.join(@mockup_dir, "*.webp"))
+        Path.wildcard(Path.join(mockup_dir(), "*.webp"))
         |> Enum.reject(&is_variant?/1)
 
       missing = Enum.reject(sources, &mockup_variants_exist?/1)